A technique for forming a thin film transistor (TFT) using a semiconductor thin film (with a thickness of approximately several nanometers to several hundreds of nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being pushed. A silicon-based semiconductor material is known as a semiconductor thin film that can be applied to a thin film transistor, and in addition, an oxide semiconductor has attracted attention as another material that can be applied to a thin film transistor.
As a material for the oxide semiconductor, zinc oxide or a material containing zinc oxide as its component is known. Further, a thin film transistor formed using an amorphous oxide (an oxide semiconductor) having an electron carrier concentration of less than 1018/cm3 is disclosed (see Patent Documents 1 to 3).
Reference
    [Patent Document 1] Japanese Published Patent Application No. 2006-165527    [Patent Document 2] Japanese Published Patent Application No. 2006-165528    [Patent Document 3] Japanese Published Patent Application No. 2006-165529